Recently, liquid chromatography-mass spectrometry devices are often used as highly sensitive means for obtaining qualitative/quantitative information on multiple components in minute amounts (order of ppm to ppb) in the fields of environment, food, pharmaceuticals, forensic medicine and the like. Thus, a device which is not only highly sensitive, which is one of the characteristics of mass spectrometry, but also simpler and highly durable and which requires simple maintenance only is desired.
One of the ionization techniques that are generally used at the interface between the liquid chromatograph and the mass spectrometer is an electrospray ion source (ESI) using the electrospray ionization technique. This is an ionization technique for generating ions by spraying a sample solution at atmospheric pressure, and one of the characteristics is that ions having information on molecular weights are generated selectively. In a liquid chromatography-mass spectrometry device using an electrospray ion source (ESI), the components in a sample mixture are separated by a liquid chromatograph, and ions are generated in an ionization unit at atmospheric pressure. Then, the ions enter a mass spectrometry unit through a first fine aperture or the like and separated according to the mass. A detection unit detects the ionic strengths, which are displayed as a mass spectrum and chromatogram data by a data processor. The types of the mass spectrometry device used in the mass spectrometry unit include a quadrupole mass spectrometer, an ion trap, a tandem quadrupole mass spectrometer, a time-of-flight mass spectrometer and the like. In general, the flow rate of the mobile phase solvent used for liquid chromatography is several hundred microliters per minute to several milliliters per minute. In order to increase the efficiency of evaporation of droplets of the sample solution sent from the liquid chromatograph and sprayed at such a high flow rate, there is a method in which heated dry gas such as N2 is blown on the droplets of the sprayed sample solution to facilitate the evaporation of the sample droplets. At this point, it is important that the droplets of the sample solution and the dry gas such as N2 are stirred thoroughly to sufficiently evaporate the droplets of the sample solution created by spraying. Accordingly, the structure for ionization becomes complex, and the ionization unit often has a large structure due to the use of gas at high temperature or the like. Because evaporation during ionization is difficult when the flow rate of the solution sent from the liquid chromatograph is high, the nano- and micro-electrospray ionization techniques in which ionization is conducted at a flow rate of several hundred nanoliters per minute to several microliters per minute for the purpose of obtaining high sensitivity are used recently. By reducing the amount of the sample sprayed and thus discharging and spraying the sample by small amounts, the use of the gas at high temperature, which is required for a high flow rate, and the ionization voltage can be reduced, and the ion source can be designed to have a small structure. However, to electrospray a sample at such a low flow rate, the opening at the end of the spraying portion has a small inner diameter of several dozen micrometers to 100 micrometers, and the opening is clogged with the sample. Thus, the spraying portion often requires frequent maintenance and replacement operations. Also, the positional relation with the inlet for the ions is important because of the small opening diameter, and it is required to adjust the positions to achieve good ionization state every time maintenance operations such as the replacement of the spraying portion are conducted. Accordingly, it is not possible yet to provide a simple, highly durable device which requires simple maintenance only which is required when the micro-electrospray ionization technique is used to increase the sensitivity.
There is a patent relating to the structure of an ion source for a low flow rate using the micro-electrospray ionization technique, as shown in PTL 1. There is a method in which the parts from the spraying portion where the sample is ionized to the fine aperture through which the ions enter the mass spectrometer are fixed in such a manner that the parts are aligned on a same line. In this method however, because the neutral molecules and the droplets in the sprayed sample solution that are not involved in the ionization are also sprayed towards the facing inlet for the ions, it is thought that there is influence of contamination on the surface of the inlet for the ions, contamination in the fine aperture and contamination in the vacuum chamber. It may be possible to increase the amount of the ions introduced, increase the signal intensities and ensure the stability by aligning the parts on a same line, but it is difficult to provide a highly durable mass spectrometer. Also, when the inside of the vacuum chamber is affected, it is required to stop creating the vacuum in the device to conduct maintenance.